1. Field of the Invention
The present invention relates to a polymer film producing apparatus and method. More particularly, the present invention relates to a polymer film producing apparatus and method in which a film surface of polymer film can be highly smooth to ensure flatness of the polymer film as product.
2. Description Related to the Prior Art
Cellulose acylates are used as a support of polymer film contained in photosensitive materials, such as photographic films, owing to advantageous characteristics, for example rigidity, non-flammability, and the like. A typical example of cellulose acylate is cellulose triacetate (TAC) having an average acetylation degree of 58.0-62.5%. Also, the polymer film of the cellulose triacetate (TAC) is used as a protection film of a polarizing element, or an optical compensation film (view angle enlarging film or the like), any of those being incorporated in a liquid crystal display (LCD) panel. This is effective because of optically utilizing the highly isotropic property of the polymer film.
A solution casting is a widely used method of producing polymer film of the cellulose triacetate (TAC) with high quality suitable for optical use. A solution casting of producing polymer film of cellulose triacetate (TAC) is typically suggested in JIII Journal of Technical Disclosure Monthly (Japan Hatsumei Kyokai, Kokai Giho), No. 2001-1745. At first, cellulose triacetate (TAC) is dissolved in a mixed solvent of which a main component is dichloromethane, methyl acetate or the like, to prepare dope or polymer solution. After this, the dope is cast on a support by a casting die, to form cast film. When the cast film dries to have a self-supporting property, a self-supporting cast film is stripped by a stripping roller. The cast film is dried, and wound as a roll of the polymer film.
To encourage drying of the cast film in the solution casting method, a dry gas flow is applied to a surface of the cast film. If application of the dry gas flow is unsuitable, it is likely to lower the surface quality of the cast film. JP-A 11-123732 discloses a suggested solution of this problem. In production of the polymer film by use of the dope or solution containing 300 wt. % or more of the solvent as a solvent content, the surface of the cast film is dried to lower the solvent content of the cast film at a level of 300 wt. % or less in drying per one minute, to increase flatness of the surface.
The cast film has an initial region from a first position of start of casting where the dope or solution from the casting die contacts the casting support to a second position of application of the dry gas flow. It is likely in the cast film that a surface is roughened by an unwanted gas flow created naturally in the initial region. Irregularity of a streak shape or patterned line shape may occur. To solve the problem, JP-A 2004-314527 discloses a gas flow regulating panel for covering the cast film in a region which is downstream from the casting die within 1,000 mm, to suppress influence of the unwanted gas flow on the surface of the cast film.
There are problems in the methods of JP-A 11-123732 and 2004-314527. Drying of the cast film according to JP-A 11-123732 is controlled at a low speed, so the productivity of the polymer film is low. As the gas flow regulating panel is suggested in JP-A 2004-314527, a gas flow will occur in a region of the gas flow regulating panel even with the gas flow regulating panel, because there is a relative speed of gas according to the run of the casting support. Attempts to keep high quality of the surface of the cast film are unsuccessful in the known techniques.